1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon processing in which a fluidized particulate catalyst is employed to effect desired reactions. More specifically this invention relates to an improved apparatus and method especially suited for carrying out fluid catalytic cracking of hydrocarbon feedstocks in such manner as to maximize the production of valuable liquid products while minimizing the production of coke and less valuable vapor products particularly the C.sub.2 and lighter products.
In modern fluid catalytic cracking units the cracking reaction can be effected by introducing the hydrocarbon feedstock at the lower end of riser reactor pipe or conduit together with hot fluidized catalyst which supplies all or a major proportion of the heat to vaporize the feedstock and carry out the endothermic reaction. The vaporized feedstock and catalyst pass up the riser reactor together at high velocity and the cracking reaction is largely complete at the top of the riser reactor. The effluent gases and catalyst enter a separator vessel where the catalyst drops to the bottom into a steam stripping zone where minor amounts of hydrocarbon associated with the catalyst particulates are stripped off. The bulk of the cracked hydrocarbons separate from the catalyst in the separator vessel and are passed out of the separator vessel through cyclones in one or more stages which separate entrained catalyst particulates. These particulates pass downwardly through the cyclone dip legs to the bottom of the separator vessel and into a steam stripper. The stripped catalyst which is convered by a deposit of coke remaining from the cracking reaction is conveyed from the stripper through a transfer line to a regenerator vessel where the coke is burned off with air or other oxygen containing regeneration gas, the heat of combustion raising the temperature of the regenerated catalyst to a level sufficiently high to provide all or a major proportion of the heat for vaporization and reaction of the hydrocarbon feed when the regenerated catalyst is conveyed from the regenerator to the inlet end of the riser reactor.
The cracking reaction is difficult to control and if the feedstock is over-cracked, valuable liquid products such as gasoline and fuel oil will be broken down into less valuable normally vapor hydrocarbons such as C.sub.2 and lighter products and excess coke. Moreover, when higher boiling feedstocks such as residual oils or heavy vacuum gas oils are cracked, higher temperatures and contact times are required to crack the principal components and undesired over cracking and coking are prone to occur particularly if the hot cracked products remain in contact with the hot catalyst after the principal desired reactions have occurred.
2. PRIOR ART
A number of conventional fluid catalytic cracking units are schematically illustrated and described on pages 113 through 120 inclusive in the September 1976 issue of Hydrocarbon Processing. The units shown on pages 114-117 have vertical riser pipes or transfer lines which extend into vessels which serve as reactors or separators or both. In the units shown on pages 118 and 119 the use of a rough cut cyclone at the upper end of the riser reactor to effect a primary disengagement or separation of the catalyst from the cracked product is disclosed. The cracked products are not conducted out of the vessel directly from this cyclone but pass into the vapor zone of the separator vessel before exiting through a second stage cyclone. U.S. Pat. No. 4,088,568, A.B.S. Schwartz, discloses the use of a rough cut cyclone in which the gaseous effluent stream is conducted to a plenum.
U.S. Pat. Nos. 4,066,533 and 4,070,159 to G. D. Myers et al, disclose rough cut cyclones directly connected to the upper extremity of a riser the top of which is open into the separator vessel. With this arrangement up flowing catalyst is carried by inertial momentum into the vessel while vapor products pass laterally into the cyclones and exit the vessel. The out flowing catalyst from the riser passes to the top of the vessel, hits a conical deflector plate which directs it radially to the sides of the vessel and finally drops by gravity to the bottom of the vessel.
The following U.S. Patents disclose the use of various kinds of flow reversal means for directing the flow of the vapors and catalyst downwardly in annular relation to the riser toward a dense catalyst bed maintained at the bottom of the vessel:
U.S. Pat. No. 3,841,843 D. P. Williams et al. PA1 U.S. Pat. No. 3,243,265 R. J. Annesser PA1 U.S. Pat. No. 3,959,117 Bunn Jr. et al. PA1 U.S. Pat. No. 4,035,284 B. Gross et al.